**Core Concept**
Niemann-Pick disease is a group of genetic disorders caused by the accumulation of sphingomyelin, a type of sphingolipid, within cells. This accumulation occurs due to the deficiency of a specific enzyme responsible for breaking down sphingomyelin.

**Why the Correct Answer is Right**
The correct answer is B. Sphingomyelinase, as it is the enzyme responsible for hydrolyzing sphingomyelin into ceramide and phosphocholine. A deficiency in sphingomyelinase leads to the accumulation of sphingomyelin within cells, resulting in the clinical manifestations of Niemann-Pick disease. This enzyme is crucial for the lysosomal degradation of sphingomyelin, and its deficiency disrupts the normal cellular process of sphingomyelin breakdown.

**Why Each Wrong Option is Incorrect**
**Option A:** Hexosaminidase is the enzyme deficient in Tay-Sachs disease, another type of lysosomal storage disorder that affects the breakdown of gangliosides, not sphingomyelin.
**Option C:** Galactokinase is involved in the metabolism of galactose, and its deficiency leads to galactosemia, a different metabolic disorder.
**Option D:** Glucosidase is the enzyme deficient in Pompe disease, a glycogen storage disorder that affects the breakdown of glycogen.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Niemann-Pick disease is classified into two main types: Type A, which is the more severe form, and Type B, which is the milder form. Type A is caused by a complete deficiency of sphingomyelinase, while Type B is caused by a partial deficiency.

**✓ Correct Answer: B. Sphingomyelinase**